There's been a lot of bad news for polar bears recently. In 2007, for example, scientists reported that if global warming continues unabated, the population could drop two-thirds by 2050. But it's not a lost cause, according to researchers who have quantified for the first time how much polar bears would benefit from reductions of greenhouse gas emissions.
Polar bears need sea ice to survive. They spend the summer on the ice hunting for seals and other prey. As the summertime extent of sea ice has shrunk in the Arctic over the past several decades, some populations have declined. In 2007, a massive reduction in sea ice raised concerns that the Arctic might have reached a tipping point, and sea ice would start to melt even more rapidly. Dwindling ice, which reflects the sun's heat, could mean that the dark waters would absorb so much heat that they would melt the remaining ice at an ever faster pace. Some media reports jumped to the conclusion that polar bears are doomed, says wildlife biologist Steven Amstrup of Polar Bears International, a small research and education nonprofit in Bozeman, Montana. Luckily, the amount of summer sea ice bounced back the following year.
Amstrup and several climate scientists decided to investigate two questions. First, does a tipping point even exist for Arctic sea ice? Second, what would be the impact of decreasing greenhouse gas emissions on the sea ice? "Although it seems reasonable to expect that reducing emissions would benefit polar bears and their habitat, no studies had been done to test whether this was actually true," Amstrup said at a telephone press conference yesterday.
The team picked five scenarios for greenhouse gas emissions and plugged them into a climate model developed by the National Center for Atmospheric Research in Boulder, Colorado. The scenarios ranged from "business as usual" (in which emissions continue to increase and the concentration of carbon dioxide reaches 689 parts per million by the end of the century) to aggressive cuts that reduce CO2 levels to the 2000 concentration of 368 ppm.
The researchers found no evidence of a tipping point that would lead to sudden loss of sea ice. Instead, all of the model scenarios showed that sea ice would decline at a steady rate as global mean annual temperature rose. (The finding is consistent with other studies , which used different methods, that have come out since Amstrup's group began its study.) "This is the first time that the issue of tipping points has been explicitly considered within the context of polar bears," says wildlife biologist Andrew Derocher of the University of Alberta in Canada, who was not involved in the research. "Clearly, the prognosis for the species is vastly better without a tipping point," he says, because a rapid loss of sea ice would be catastrophic for the species.
To figure out what reductions in greenhouse gas emissions would mean for the fate of the polar bears, the researchers compared the rates of sea ice loss under the various scenarios. As they report  online today in Nature, the business-as-usual scenario led to a 50% loss of optimal polar bear habitat (sea ice must be not too thin and not too thick) by midcentury. Under the mitigation scenario, only 20% of the ice was lost. By combining this result with other data, such as measurements of how long the best sea ice lasts each year, Amstrup's team was able to estimate how much benefit would accrue to polar bears in four large regions.
Under the business-as-usual scenario, the chance of polar bears vanishing from these regions ranges from about 50% to roughly 80% by midcentury, depending on the region. In contrast, if emissions are reduced, the chance of extinction is negligible for polar bears in two regions, and 25% to 50% for the other two. "Conserving polar bears appears to be largely a matter of minimizing temperature rise," Amstrup says. And if management of the species is also optimized—by minimizing hunting and disruptions from oil and gas extraction—polar bear populations are likely to increase in northern Canada and Greenland.
That's a much brighter picture for polar bears, if emissions can be brought under control. "One of the reasons why I believe this paper is so important is that it corrects the notion that caught hold in the media and some parts of the public that there was nothing that could be done to save polar bears from extinction," says Derocher.